Voltage comparison circuits are used in many different applications that are well known to those skilled in the art. One application for voltage comparison circuits is to detect the sensed output voltage from a geartooth magnetic sensor that utilizes a plurality of magnetoresistors connected in a Wheatstone bridge arrangement. Typical MR bridge sensors employ four magnetoresistors arranged in a bridge configuration so that the voltage differential across the bridge is indicative of the presence or absence of a magnetic component, such as a geartooth, in a predefined detection zone proximate the geartooth sensor.
FIG. 1 is a high-level block diagram illustrating a conventional sensor system 10 such as may be utilized for geartooth or other magnetic sensing applications. Sensor system 10 generally comprises a magnetic signal sensor 2 having an output coupled to a signal conditioning module 4. Magnetic signal sensor 2, which may be a magnetoresistive (MR) bridge, a Hall effect device, or other non-contacting magnetic sensor, includes magnetically sensitive elements for detecting a varying magnetic field proximate to the sensor and translating the detected field into a real time differential voltage output representative of the relative position of the specified target. Signal conditioning module 4 receives the differential output from sensor 2 and includes circuit components and devices for translating the sensor signal into a suitable digital format that may be processed by a sensor microcontroller 12 to determine and provide control function signals related to the angular or linear motion of the specified target.
Signal conditioning module 4 includes a differential amplifier 6 that pre-amplifies the analog sensor signal, and a sensor output detector module 8, which digitizes the amplified signal. Detector module 8 typically includes comparator functionality for comparing the incoming amplified analog signal with one or more specified threshold levels to determine the digital switching points. Referring to FIG. 2A in conjunction with FIG. 1, a waveform representation of the amplifier output signal, Vs, is depicted in relation to the switching point thresholds employed by detector module 8. Ideally, and as illustrated in FIG. 2A, Vs is centered at the 0 volts reference level to enable accurate signal detection.
The zero offset of the two different amplitude sine waves Vs1 and Vs2 cross at the same 0v reference points. Setting the switch points +SP and −SP closer to the 0v point, would minimize the switch point timing error resulting from the amplitude difference between Vs1 and Vs2. Even with switch points that are a substantial fraction of the signal amplitude as shown in FIG. 2A, the output signals from the detector Vout1 and Vout2 both have a 50% duty cycle and exhibit some phase shift. However, as shown in FIG. 2B, a DC offset is often imparted as on the depicted sensor signals Vs1 and Vs2 from a variety of sources including temperature and component calibration and tolerances of sensor 2 and amplifier 6.
In order to maintain the amplified signal in a detectable range to ensure reliable detector switching, the detector switch points +SP and −SP must have a sufficient range from the zero reference level to account for the DC offset. The DC offset may be of either polarity and often results from small mismatches in components which are designed to be as similar as possible such as the sensor bridge and input differential amplifier.
A problem relating to reliable switch point detection arises, however, when a DC offset is introduced in a reduced amplitude sensor signal. Namely, FIG. 2B illustrates a first signal, Vs1, representative of an expected amplitude signal, and a second signal, Vs2, representing a sensor signal that has been attenuated such as by an environment factor such as an increased air gap between the sensing element and target object. As seen in the depicted waveform comparison, a DC offset has been introduced that is large enough so that as Vs2 goes negative, the detector doesn't switch since the downward amplitude peak of Vs2 remains less than the offset added to the predetermined switch point. For smaller values of offset, switching will occur but the switching will move away from a 50% duty cycle as shown in FIG. 2B for Vout1.
By eliminating the DC offset, AC coupling of the sensor signal improves switch point detection reliability and reduces the need for a wider switch point span, thus enabling the switch points for detector module 8 to be set substantially near the ideal reference level. A variety of AC coupling techniques are known and are utilized in signal measuring and detection applications for enabling measurement of AC signals riding DC offset levels. The most common technique involves the use of a series coupled capacitor located between a signal input and the first amplification stage. The pre-amplification coupling can be necessitated in practice, in order to prevent the amplifier gain from disturbing the quiescent state of the AC coupling capacitor.
If signal gain was switched ahead of the coupling capacitor, the DC value applied to the capacitor would also change and the circuit would require a relatively long time to settle. Furthermore, a series AC coupling capacitor design is susceptible to stray electromagnetic interference (EMI) and non-linearities caused, for example, by the additional lead contacts required for the series coupled capacitor. These stray effects can degrade system performance by introducing RC time constants that limit the circuit's bandwidth or by introducing distortion components.
Given that a significant DC offset component may be imparted by the differential amplifier 6 as well as sensor 2, the pre-amplifier stage coupling requirement of an inline AC coupling capacitor renders this approach unsuitable for addressing the DC offset problems encountered by a conventional sensor system such as magnetic sensor system 10.
An alternative AC coupling technique employs a so-called DC buckout circuit. In a DC buckout circuit, a specified DC voltage is subtracted from the input signal, leaving a non-offset AC component. However, buckouts circuit performance is degraded by temperature drift of the subtraction voltage with temperature and other operating environment conditions, and drift of the DC component sought to be removed, both of which result in a remaining DC component.
An AC coupling technique well-suited to application within a magnetic sensor system such as magnetic sensor system 10 is disclosed in U.S. Pat. No. 6,657,476, the content of which is incorporated by reference herein in its entirety. The AC coupled sensor signal conditioning device disclosed in U.S. Pat. No. 6,657,476 utilizes a low-pass filter comprising a holding capacitor at the threshold input of a comparator device. While effective for eliminating the DC offset contributed by the sensor and the preamplifier, the AC-coupled conditioning circuit disclosed by U.S. Pat. No. 6,657,476 requires a significant startup time delay during which the holding capacitor initially charges. When implemented in a geartooth sensor, for example, such a startup delay may result in the sensor failing to track geartooth targets during system startup.
It would therefore be useful to address problems relating to DC offset in sensor signal conditioning circuits without compromising reliable sensor output during system startup. In view of the foregoing, a need remains for a sensor signal conditioning circuit that eliminates DC offset in the post amplification stage while maintaining an accurate and reliable sensor output tracking during system startup. The present invention addresses such a need.